Liquid crystal compounds are used in human and machine readable displays finding applications in all types of instrument controls, e.g., in motor vehicles, avionics, medical devices, process control devices; in timing devices, e.g., watches; etc. Display devices are primarily comprised of liquid crystal cells having a glass or other substrate coated with a transparent conductive material in front and behind a liquid crystal medium. Light transmission through these devices is controlled through orientation of the liquid crystal compounds or dyes dissolved therein. That is, a voltage or, in some instances, a magnetic field may be applied to the cell so that the liquid crystals and, where present, dyes (collectively referred to as liquid crystal medium) are oriented in a fashion such that all, some or none of the light is passed through. In addition, depending on the device geometry, one or two polarizers may be used in conjunction with the liquid crystal medium to control light transmission. A composition comprised of one or more liquid crystal compounds having one or more dyes dissolved therein is generally referred to as a "guest-host" system.
Liquid crystal compounds useful in these displays are anisotropic. That is, they (a) exhibit properties with different values when measured along axes in different directions and (b) assume different positions in response to external stimuli, e.g., an applied voltage. More specifically, they are birefringent. An applied voltage can be used to rotate the liquid crystals along a fixed axis so as to alter their optical properties. This phenomenon can be used to modulate light.
Dyes useful in "guest-host" systems are also anisotropic. That is, they exhibit dichroism (have different light absorption characteristics along different axes). Dichroic dyes best suited for displays absorb more light along one axis and absorb less light along a second. When the liquid crystal host composition and guest dye are properly matched, such a dye may be dissolved in the liquid crystal composition and the transmission characteristics of the mixture can be controlled through an applied voltage. The voltage is applied in a manner so as to place the mixture in a position such that only the desired amount of light will be transmitted through the dye.
All aligned liquid crystal cells in commercial use today are oriented in directions suitable for controlling light transmission. That is, the liquid crystal composition or guest-host mixture is aligned so as to assume a homogeneous or homeotropic alignment. Without external stimuli the display will either appear opaque or transparent. By applying an electric field the molecules are rotated along a fixed axis so as to alter the transmission properties in a desired fashion.
Many techniques are known for aligning liquid crystal media. Typically, with homogeneous alignment, the inner surfaces of the glass substrates are coated with an alignment layer, e.g., a polymer film, and the coating is rubbed along a fixed linear direction with a cloth. By rubbing the surface, one preferentially alters the surface state so that generally the "long" molecular axis of the liquid crystal aligns along or relative to the rubbing direction. This alignment is transferred to the bulk of the liquid medium via intermolecular forces.
A homeotropic alignment is characterized by the long axis of the liquid crystal aligning along or relative to an axis perpendicular to the glass substrate. Typical alignment layers, applied to the glass substrate, are comprised of polyalkylsiloxanes and lecithins.
A simple technique for aligning or realigning liquid crystal and guest-host media has been desired. In addition, it has been desired to eliminate the alignment layers used in liquid crystal displays and processes necessitated by their use in order to decrease cost and improve performance of the displays. Further, a liquid crystal cell having two or more regions aligned in different homogeneous or homeotropic alignments has been desired. These objectives can be achieved using linearly polarized light per the claimed invention.